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Message: [QUOTE="Charles, post: 678008, member: 103"] Since I make the exact same run over and over and over and over, it's very obvious that the little ZF can't handle the heat. I melted my first one in my 250 at a dyno verified 305rwhp with a steady-state boost of ~24lbs. I melted my second in the 550 at about the same power and about the same boost. The 550 made two runs before it melted. It was in OD with 4.88 gears whereas my 250 was in direct with 3.73's, both at about the same ground speed (70 to 75). I don't know if the 550 trans lasted longer because of it being in OD or if that didn't matter, don't care. Neither would remain in gear with any level of success. You could keep driving, as long as you didn't mind shoving the shifter as hard as you could into gear under power and sometimes still have it go BAAAAAM, and jump out of gear under load. Since that's not my idea of acceptable I moved on to another trans in each case. My truck got another ZF, which I melted again, and then a 4R, and the 550 got another ZF with an added cooler inline with the OEM radiator unit.... maybe that will help... Meanwhile, my 250 has since made that run with the 4R at a steady-state boost of about 45lbs!!! In a program dyno verified at 635rwhp, and the fluid looked exactly the same when I got home. It can shed the heat. And in that case... it was handling much, much more power. I had 4 more bales on the trailer and was running 75 the whole time, even cresting the hills because I had so much power reserve and two stage water injection keeping the engine happy and cool. The 4R gave not a single ***. Explain how Ford was going to put a ZF 650 rated at 650ft/lbs in a truck with an engine rated at 800ft/lbs and still have a warranty from ZF? Ever wonder why the last trucks to receive the ZF were making exactly 650ft/lbs? It's because they couldn't dial them up any more than that without killing the pos trans. Sure they would hold for short bursts, but Ford has to warrant the truck for it's intended purpose, TOWING AT RATED POWER. If you took a bone stock 6.7 at 400hp and went out and started dragging trailers around at full power you would end a pos ZF6, and ZF would have denied the warranty, leaving ford to foot the bill. That is why there is no more ZF6. The engine's left it behind. The pinnacle of concrete delivery are the oshkosh trucks. Allisons in 100% of them. They would run clean through a pos rear discharge with an eaton trans while carrying 10 yrds of concrete and never know it. Believe it or not, dumb asses not being able to easily work on something is a big time barrier to a lot of things. I'm not talking about confusing the people driving it, I'm talking about the people working on it. When your mechanic tells you he can't work on an auto, but he can swap a dry clutch all day in front of a manual, that leads to a lot of people being scared of buying a truck with an auto. I'm doing my best to supply real info, as opposed to feel-good bs. [/QUOTE]